In the past several decades, many applications have been developed that require measurements of electrical currents. One convenient technique for measuring a current flowing through a conductor is to insert a small shunt resistor into the current path. The shunt resistor generates a voltage, which is proportional to the amount of current flowing through the shunt resistor. The voltage can be measured by a voltage measurement instrument and used to identify the amount of current. The shunt resistor and the voltage measurement instrument could therefore represent a current measurement instrument.
One advantage of this technique is that the value of the shunt resistor can be modified to adapt the range of the current measurement instrument to the range of currents expected in a particular application. While some power can be dissipated in the shunt resistor, the power loss can be reduced or minimized by reducing the value of the shunt resistor. This, however, reduces the voltage across the shunt resistor and puts more stringent demands on the current measurement instrument.
The current measurement instrument used in this technique often has to measure voltages that are relatively large compared to the supply voltages of modern integrated circuits. For example, a current being measured may be associated with a higher-voltage application (such as 12V, 24V, or even higher). In contrast, a component that processes the current measurements often operates at a lower voltage, such as when a signal representing the current measurements is provided to an analog-to-digital converter (ADC) that uses a supply voltage of 5V, 3.3V, or less. The current measurement instrument therefore needs to amplify a small differential voltage across a shunt resistor and transform it into a signal that is typically ground-referenced (though it could be differential or referenced to another voltage level). At the same time, the current measurement instrument may need to reject a potentially large common mode voltage.
One type of current measurement instrument that can perform these functions is a current sense amplifier. A current sense amplifier generally represents a voltage difference amplifier that is designed for sensing the differential voltage across a shunt resistor while rejecting large input common mode voltages. Current sense amplifiers typically have a voltage gain of between ten and one hundred (although other voltage gains could be used). Also, current sense amplifiers typically have an input common mode voltage range (CMVR) that extends far above, and sometimes also below, their supply rails.
While various types of current sense amplifiers have been developed over the years, they often suffer from various problems. For example, current sense amplifiers implemented using resistive dividers often suffer from offset, noise, and speed/bandwidth problems, and they often require precise trimming of the resistors in the resistive dividers. Other current sense amplifiers have topologies implemented in high-voltage fabrication process technologies. However, these current sense amplifiers are often large, resulting in increased die sizes and higher costs.